Patent Application
20060057377
1. Field of the Invention
The present invention relates generally to the production of electroactive polymers via electrospinning. It relates in particular to the induction of the polar phase in electroactive polymers, and the spontaneous orientation of dipoles in a single step process. These materials have widespread application in numerous fields, including aerospace, biomedical, clothing, and defense.
2. Description of the Related Art
Due to their exceptional thermal, mechanical, and dielectric properties, polyimides are widely utilized, such as for matrix materials in composite aircraft components and as dielectric materials in microelectronic devices. Further, electroactive polymers have properties that are advantageous in numerous fields. Unfortunately, current methodology for producing electroactive polymers entails melt-pressing of the polymer at high temperatures, followed by stretching and corona-poling at high electric field strengths in order to produce the polar phase and electroactive behavior. This current process is labor-intensive and time-consuming, and requires large equipment that is complex to maintain.
Currently, polyvinylidene fluoride (PVDF) is the only commercially available piezoelectric polymer. PVDF is ferroelectric, having a polar axis that can be reoriented when an electric field is applied, and piezoelectric, when subjected to mechanical stress, changing its electrical polarization, or vice versa, with a change in electrical polarization resulting in mechanical movement. PVDF exhibits piezoelectricity after being subjected to a poling process which applies a high electric field to force molecular dipole alignment. PVDF can exist in several solid state phases, α, β, δ, and γ. The β-phase is of most importance because it is this phase that shows the largest electroactive response. The different phases can be attained by application of mechanical, thermal, or electrical stress depending on the initial and desired state of the polymer. The process that leads to the β-phase formation is labor intensive and time consuming, requiring initial melt processing and drawing into a film, followed by further stretching of the film at an elevated temperature either uniaxially or biaxially to induce the polar phase. Finally, it is passed under corona wires at high voltage to cause the induced dipoles to orient. PVDF has the potential to be used in many applications due not only to its electroactive properties but also because it is lightweight, flexible, tough and conformable. In addition, it possesses significant resistance to fatigue, abrasion, deformation, chemicals, and solar radiation. These properties make it an attractive material for aerospace as well as medical applications.
In the fiber-spinning process known as electrospinning, a high voltage is applied to a polymer in solution to create nanofibers and nonwoven mats. The polymer solution is loaded into a syringe, and high voltage is applied to the needle of the syringe. Charge builds up on a droplet of solution that is suspended at the tip of the syringe needle. Gradually, as this charge overcomes the surface tension of the solution, this droplet elongates and forms a Taylor cone. Finally, the solution exits out of the tip of the Taylor cone as a jet, which travels through the air to its target medium. While traveling, the solvent evaporates, leaving fibers. Overall, electrospinning is advantageous for many reasons. It is simple, and the technique is fast and easy to run. Only a small amount of material is required, and there is very little waste. The products of this process also have advantages over currently available materials; the fibers are very thin and have a high length to diameter ratio, which provides a very large surface area per unit mass. Finally, the process is versatile. Fibers can be spun onto any shape using a wide range of polymers. While electrospinning is an advantageous processing method to apply to polymers, it has not been applied for the purpose of producing electroactive polymers. An additional advantage of electrospinning is the ability to produce fibers and fibrous nonwoven mats; current methods for the production of electroactive polymers generally produce films.
While electroactive polymers themselves embody many useful properties, doping electroactive polymers with inclusions, such as nanotubes, nanofibers, and piezoceramic powders for dielectric enhancement is advantageous. Carbon nanotubes have become increasingly interesting due to their very unique properties: high tensile strength and modulus, high electrical conductivity, and high thermal conductivity.
It is accordingly a primary object of the present invention to overcome the difficulties and avoid the inadequacies presented by existing processes for the production of electroactive polymers. The present invention produces electroactive polymers via electrospinning. The induction of electroactivity via electrospinning can be utilized with one or more soluble polymers with polarizable moieties. Suitable polymer classes include but are not limited to polyimides, polyamides, vinyl polymers, polyurethanes, polyureas, polythioureas, polyacrylates, polyesters, and biopolymers. The polyimides include but are not limited to 2,6-bis(3-aminophenoxy)benzonitrile ((β-CN)APB)/4,4′oxydiphthalic anhydride (ODPA) ((β-CN)APB-ODPA) and an amorphous polyimide such as amorphous polyetherimide (such as the commercially available Ultem®). The polyamides include but are not limited to odd-numbered nylons. The vinyl polymers include but are not limited to PVDF, PVDF/TrFE (copolymer of vinylidene fluoride and trifluoroethylene), poly(vinyl alcohol) (PVA), a graft elastomer such as that claimed in U.S. Pat. No. 6,515,077, and vinyl copolymers. The polyacrylates include but are not limited to poly(methyl methacrylate) (PMMA). The biopolmers include but are not limited to polypeptides and keratin. Any one or more solvents sufficient to dissolve the one or more polymers of interest and make a spinnable solution can be utilized. Suitable solvents include but are not limited to N,N-Dimethylformamide (DMF), N,N-Dimethylacetamide (DMAc), N-methylpyrrolidinone (NMP), toluene, and cosolvents including but not limited to DMF/acetone. The polymer can be electrospun into fiber and fibrous nonwoven mat forms. The availability of electroactive polymer fibers and fibrous nonwoven mat will enable many new applications for electroactive polymers.
The present invention allows for the production of electroactive polymers using a simple set-up and a processing method that is fast and easy to run. Additionally, electroactive fibers can be created that are only nanometers in diameter. In situ induction of polar phase and spontaneous dipolar orientation of electroactive polymers by a single step electrospinning process produces electroactive polymers from a polymer solution. The need for direct contact or corona filed poling is eliminated, resulting in arc-free processing. Further, nanofibers and fibrous nonwoven mats are produced with minimal pre- and post-processing. Enabling materials are provided for a wide expanse of applications in such fields as aerospace, biomedical, military and environmental.
Additional objects and advantages of the present invention are apparent from the drawings and specification which follow.
According to the process of the present invention, a solution is provided which comprises one or more polymers having polarizable moieties dissolved in a solution. Polymers with polarizable moieties have asymmetric strong dipoles. Suitable polymer classes include but are not limited to polyimides, polyamides, vinyl polymers, polyurethanes, polyureas, polythioureas, polyacrylates, polyesters, and biopolymers. The polyimides include but are not limited to 2,6-bis(3-aminophenoxy)benzonitrile ((β-CN)APB)/4,4′oxydiphthalic anhydride (ODPA) ((β-CN)APB-ODPA) and amorphous polyimides such as amorphous polyetherimide (such as the commercially available Ultem®). The polyamides include but are not limited to odd-numbered nylons. The vinyl polymers include but are not limited to PVDF, PVDF/TrFE (copolymer of vinylidene fluoride and trifluoroethylene), poly(vinyl alcohol) (PVA), a graft elastomer such as that claimed in U.S. Pat. No. 6,515,077, and vinyl copolymers. The polyacrylates include but are not limited to poly(methyl methacrylate) (PMMA). The biopolmers include but are not limited to polypeptides and keratin. Any one or more solvents sufficient to dissolve the one or more polymers of interest and make a spinnable solution can be utilized. Suitable solvents include but are not limited to N,N-Dimethylformamide (DMF), N,N-Dimethylacetamide (DMAc), N-methylpyrrolidinone (NMP), toluene, and cosolvents including but not limited to DMF/acetone.
Doping the one or more polymers with inclusions, such as nanotubes, nanofibers, and piezoceramic powders for dielectric enhancement would be advantageous. The inclusions can be induced prior to electrospinning using processes known in the art.
Referring now to the drawings, and more particularly to
The spun product is allowed to dry until solvent-free. Drying time should be sufficient to allow solvent evaporation to occur. Drying may occur in a vacuum oven or at room temperature.
In an alternate embodiment, referenced generally in
An additional embodiment of the electrospinning apparatus includes the incorporation of a heater to regulate temperature of the electrospinning environment.
The polymer solution is electrospun to produce fibers of approximately 10 nm to approximately 10 μm in diameter, as well as fibrous nonwoven mats of customizable size, shape, fiber orientation, and thickness. Depending on the spun product desired, the collector 160 is designed accordingly. Parameters in the processing include infusion rate, applied voltage, collector's material and rotation and/or translation speed, distance between the needle 140 tip and collector 160, and drying time.
Parameters sufficient to form dry, uniform diameter solid fibers are preferred, and are customized based on the polymer of interest. The concentration of polymer in solution generally correlates with fiber size, with lower concentrations producing smaller diameter fibers. Additionally, the size and proportion of fibers versus droplets generally increase with increase in concentration. Voltage will generally be in the approximate range of 5 to 30 kV, and is adjusted to achieve the volume and diameter of fibers desired. Distance between the needle 140 tip and collector 160 will generally be 3-10 inches. It is desirable to have an infusion rate that delivers the appropriate amount of solution, with balancing of the infusion rate and applied voltage to maintain a pendant drop of solution at the tip of the nozzle. Infusion rates will generally be approximately between 3 and 10 mL/hr.
The following examples are illustrative of the present invention, and are not intended to limit the ambit thereof.
PVDF pellets (MW 530,000, Aldrich Chemical Company, Inc.) were dissolved into solvent DMF at a concentration of 30 weight percent (wt %) PVDF. The solutions were electrospun into fibers using the apparatus illustrated in
Differential Scanning Calorimetry Analysis (DSC)
Differential scanning calorimetry (DSC) measurements were completed using a Perkin Elmer Pyris 1 calorimeter. Film and electrospun mat samples ranged in mass from 3.0 to 6.0 milligrams (mg); PVDF pellet and commercial film samples were 5.0 to 11.4 mg. The thermal program consisted of heating from −65° C. to 250° C. at a rate of 10° C./min. Heat flow was recorded for all samples. Melt temperature (Tm), defined as the temperature at the maximum of the endotherm, and heat of fusion (ΔH), defined as the area of the melt peak, were determined from the first heat.
DSC analysis was used to indicate degree of crystallinity and specific crystal form present in the PVDF samples. Table 1 summarizes the DSC results for β-phase PVDF (commercially-purchased stretched and poled film), α-phase PVDF (commercially purchased pellets), and electrospun fibers. It was observed that the value of the main melting point Tm for the 15 kV and 20 kV electrospun fibers was higher than that of the pellets and closer to that of the commercial PVDF films. Tm of the 10 kV electrospun fibers was similar to that of the α-phase pellets. This indicates that the 15 kV and 20 kV electrospun fibers exhibit β-phase. The same was true for the ΔH, where all three electrospun cases showed values comparable to the stretched and poled β-phase commercial films. DSC results for the three spun cases are summarized in
*Shoulder on lower side of Tm.
As illustrated in
X-Ray Diffraction Analysis (XRD)
X-ray diffraction (XRD) was performed using a Philips Analytical X'Pert Pro X-ray Diffraction System with a step size of 0.0080° 2θ and scan step time of 120 seconds. The range of interest for the measurements was for 2θ between 10 and 40°. The phases present in the films were determined by examining the characteristic absorption bands of the respective crystalline phases.
An assessment of the degree and type of crystallinity of the three electrospun samples using XRD data supports the findings of the DSC data.
XRD data clearly confirm the DSC results. The presence of a shoulder on the low end of the melt peak seen in DSC corresponds to the finding by XRD that a second crystalline form is present in the electrospun fibers. This shoulder is attributed to β-form. In mixed systems, β form has smaller reflections than α-form, which are dominant, so detecting small amounts of β-form becomes difficult.
Fourier Transform Infrared Spectroscopic Analysis (FTIR)
Infrared spectroscopy (IR) was performed on the samples using a ThermoNicolet IR 300 Spectrometer. Data analysis was performed with Omnic version 6.2 software. The measurements were taken from 600-1500 wavenumbers (cm−1) with 128 scans performed per sample. Data analysis consisted of qualitative visual comparison of intensities of characteristic transmittance peaks for PVDF crystalline phases.
Thermally Stimulated Current Analysis (TSC)
A SETARAM TSC 3000, automated Thermally Stimulated Current (TSC) equipment, was used to track the relaxation processes in the polymeric films and fibers. Heating of the sample at a constant rate accelerates the real charge decay, which can be observed as a current release. The current as a function of temperature was measured by a sensitive electrometer connected to the electrodes. No additional poling was performed on any of the samples between initial electrospin processing and TSC analysis. All samples were subjected to the same cycle. The samples were heated from 25° C. to 150° C. at a rate of 2° C./min.
Thermally Stimulated Current (TSC) measures the release of stored dielectric polarization in the form of charge or current. Because piezoelectricity in PVDF arises from orientation polarization of the —CF2— dipoles in the polar phase, TSC analysis can be used to reveal the presence of this orientation polarization current peak. The peak was shown to shift from 90° C. to 130° C. as more perfect and thermally stable crystallites are formed [13]. A second peak, due to space charge release, typically occurs at higher temperatures.
To assess the potential piezoelectricity of the 15 kV fibers, the pyroelectric coefficient, p, was calculated using the following equation:
where I is the depolarization current, A is the area of the electrode, and dT/dt is the heating rate during TSC. The pyroelectric coefficient was measured with respect to current released up to 140° C., since the peak of interest is the orientation polarization peak located below this temperature. The measured pyroelectric coefficient was 1.5×10−5 C./° C.−m2 for the 15 kV fibers. Values for stretched, poled PVDF are typically in the range of 2.0−3.5×10−5 C./° C.−m2.
PVDF solutions in the solvent DMF with and without single wall nanotubes (SWNTs) were electrospun at various concentrations, as outlined in Table 2. PVDF pellets (MW 530,000, Aldrich Chemical Company, Inc.) were dissolved into DMF at a concentration of 30 weight percent (wt %) PVDF. A SWNT stock solution (1% w/w) in DMF was prepared to mix with PVDF/DMF solution to prepare various compositions of SWNT/PVDF/DMF solutions. The solutions were electrospun into fibers using the apparatus illustrated in
The varying concentrations of PVDF and PVDF with SWNT shown in Table 2 were spun onto Indium-Tin Oxide (ITO) coated glass plates and standard glass microscope slides utilizing the electrospinning apparatus illustrated in
The 30 wt % PVDF was also spun onto a rotating roller to produce an oriented fibrous mat, using the apparatus of
Once spun, on either plates or the roller, optical microscopy was completed at 100× and 200× magnification using an Olympus BH-2 optical microscope in conjunction with Scion Image, v. 1.62, software. These images were utilized to compare and observe trends as a function of electrospinning parameters.
Mats spun on a roller were dried in a vacuum oven at 60° C. overnight or over a weekend. The vacuum oven removed any remaining DMF solvent from the fibers. Solvent vapors were allows to flow over dry ice, were condensed into liquid form, and were then contained in a liquid trap. Once dried, the PVDF mats were prepared for Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), and Dynamic Mechanical Analysis (DMTA) by cutting to appropriate size.
The concentration of PVDF in DMF solution correlated with fiber size. The lowest concentration, 15 wt %, produced wet fibers with a small diameter. The size and proportion of fibers versus droplets produced increased as concentration was increased, as shown in
Various electrospinning parameters were examined for the 30 wt % PVDF in DMF solution to determine the optimal conditions. Voltage was varied from 5 kilovolts (kV) to 30 kV. As is illustrated in
Infusion rate was not a dramatic factor in determining fiber morphology. However, in choosing an infusion rate, it was desirable to have a rate that delivered the appropriate amount of solution. The infusion rate must be balanced with applied voltage to maintain a pendant drop of solution at the tip of the nozzle. In addition, the appropriate infusion rate varied with solution viscosity. The infusion rate was varied from 3 to 10 mL/hr for 30 wt % PVDF. As can be seen in
Optimal conditions for 30 wt % PVDF were found to be a voltage of 10 kV, an infusion rate of 6 mL/hr, and a distance of 9 inches from the needle point to the collecting plate.
PVDF combined with carbon nanotubes was tested beginning with 0.1% SWNT in 15 wt % PVDF and 1.0% SWNT in 15 wt % PVDF. Small and few fibers formed. When the 1.0% SWNT was electrospun, black drops fell out of the solution as a result of the excessive concentration of SWNTs. A 0.1 % SMNT in 25 wt % PVDF spun well and was able to be spun onto a rotating roller, producing a nonwoven mat which was vacuum-dried at 60° C. A nonwoven mat was also produced from 0.2% SWNT in 25 wt % PVDF. Optimal conditions for the PVDF solutions with carbon nanotubes were slightly different depending on amount of carbon nanotubes and concentration of PVDF.
Although the present invention has been described relative to specific embodiments thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.
Pursuant to 35 U.S.C. § 119, the benefit of priority from provisional application having U.S. Ser. No. 60/530,637, filed on Dec. 19, 2003, is claimed for this nonprovisional application.
The invention described herein was made in part by employees of the United States Government and may be manufactured and used by and for the Government of the United States for governmental purposes without the payment of any royalties thereon or therefore.
| Number | Date | Country | |
|---|---|---|---|
| 60530637 | Dec 2003 | US |
